If a player sacrifices their commander this way, they won't be able to choose whether to put it into the command zone until after Danse Macabre has finished resolving. Notably, this means that the sacrificed commander is a legal choice for the spell's controller to return to the battlefield. If they choose it this way, it will stay on the battlefield under their control.

You add only the toughness of the creature you sacrificed this way, not the toughness of any creatures your opponent sacrificed. If you didn't sacrifice a creature this way, you will still roll a d20, but you will not add anything to the roll.

Starting with you and continuing in turn order, each player will choose which creature they will sacrifice. They will know the choices made by players before them in this order when they make that choice. The creatures are all sacrificed at the same time.

All creatures are chosen and sacrificed before the die is rolled. You don't get to see what the result of the die roll is before choosing which creature to sacrifice.